I've always loved the idea of putting together many smaller computers to make a 'personal grid' or cluster, partly out of sheer interest but also because I do some grid work and having a small grid of individual machines in a relatively small case would be an easy way to test things out.

I always liked the BriQ, although it was expensive. Now there's a cheaper Xscale CPU based unit from Team ASA in the form of the catchily titled NPWR SCE.